Nucleosome

Definition

A nucleosome is the fundamental unit of DNA packaging in eukaryotic cells, consisting of a segment of DNA wrapped around a core of histone proteins. This structure plays a crucial role in organizing and compacting DNA, allowing it to fit within the cell nucleus while also playing a key role in gene regulation and expression. Nucleosomes are essential for maintaining genomic integrity and facilitating processes like DNA replication and repair.

5 Must Know Facts For Your Next Test

1. Nucleosomes consist of about 147 base pairs of DNA wrapped around an octamer of histone proteins, which includes two copies each of histones H2A, H2B, H3, and H4.
2. The arrangement of nucleosomes along the DNA creates a 'bead-on-a-string' structure, which can further fold into higher-order chromatin structures.
3. Nucleosomes play a vital role in regulating gene expression by controlling the accessibility of transcription factors to specific DNA regions.
4. The addition or removal of chemical modifications on histone tails can influence how tightly or loosely DNA is packaged, affecting gene activity.
5. Nucleosomes are involved in processes like DNA replication and repair, ensuring that the genetic material is properly organized and maintained during cell division.

Review Questions

• How do nucleosomes contribute to the organization and regulation of DNA within eukaryotic cells?
  • Nucleosomes help organize DNA by wrapping it around histone proteins, creating a compact structure that fits within the cell nucleus. This organization not only allows for efficient packaging but also plays a critical role in regulating gene expression by controlling access to DNA. When nucleosomes are tightly packed, transcription factors may have difficulty binding to DNA, thereby downregulating gene activity. Conversely, when nucleosomes are spaced further apart, genes can be more easily accessed for transcription.
• Discuss the impact of histone modifications on nucleosome structure and function in gene regulation.
  • Histone modifications, such as methylation or acetylation, can significantly alter nucleosome structure and function. For instance, acetylation typically leads to a more relaxed chromatin structure, making the associated genes more accessible for transcription. In contrast, methylation often results in tighter packing of nucleosomes, silencing gene expression. These modifications act as epigenetic markers that influence how genes are expressed without changing the underlying DNA sequence.
• Evaluate the role of nucleosomes in cellular processes such as DNA replication and repair, and their broader implications for genomic stability.
  • Nucleosomes play an essential role in both DNA replication and repair by ensuring that the genetic material remains organized and accessible throughout these processes. During DNA replication, nucleosomes must be disassembled to allow for copying the DNA strands; after replication, they are reassembled to restore proper packaging. Similarly, when DNA damage occurs, nucleosomes must be temporarily altered to facilitate repair mechanisms. This dynamic nature of nucleosomes helps maintain genomic stability by allowing necessary changes while protecting against potential mutations.

Related terms

histone: Histones are small, positively charged proteins that play a critical role in the structure of nucleosomes by forming the core around which DNA is wrapped.

chromatin: Chromatin is a complex of DNA and proteins, primarily histones, that makes up the chromosomes in eukaryotic cells, existing in two forms: euchromatin (less condensed) and heterochromatin (more condensed).

DNA supercoiling: DNA supercoiling refers to the over- or under-winding of the DNA double helix, which occurs as a result of the compact organization of DNA into structures like nucleosomes.